Exploration of the origin of large first hyperpolarizabilities of trisaza-bridged (36) fulleroids.

We investigate theoretically the structures and second-order nonlinear optical (NLO) responses (first hyperpolarizabilities) of 15 trisaza-bridged (36) fulleroids (series-A) and 15 triborane-bridged (36) fulleroids (series-B). 3A has smaller transition energy and smaller ground state dipole moment, resulting in relatively larger static first hyperpolarizability (10647 au). Most trisaza-bridged (36) fulleroids have larger β values than the corresponding triborane-bridged (36) fulleroids. The f(0) and Δμ remain stable values when substituents R change for series-A except 2A and 5A (for series-B except 5B and 10B) and β values are proportional to ΔE(-3), which implies that the β values for series-A and series-B follow the two-level model. Results demonstrate that a proper bridge and lower transition energy ΔE are more favorable to enlarging first hyperpolarizabilities of series-A and series-B. In addition, the frequency-dependent SHG and EOPE are also estimated and discussed. The current work can stimulate experimentalists to synthesize novel NLO materials designed in this work.